Development Application Part 2 of 2 Planning report Geotechnical report and plans MCU11/0168 & OPW11/0257

Tree root growth
Trees and shrubs that are allowed to grow in the vicinity of footings
can cause foundation soil movement in two ways:

• Roots that grow under footings may increase in cross-sectional
     size, exerting upward pressure on footings.

•    Roots in the vicinity of footings will absorb much of the moisture
     in the foundation soil, causing shrinkage or subsidence.




    Unevenness of Movement

The types of ground movement described above usually occur
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unevenly throughout the buildings foundation soil. Settlement due
to construction tends to be uneven because of.

•     Differing compaction of foundation soil prior to construction.
•     Differing moisture content of foundation soil prior to construction.
                                                                                 As the weather pattern changes and the soil begins to dry out, the
Movement due to non-construction causes is usually more uneven                   external footings will he first affected, beginning with the locations
still. Erosion can undermine a footing that traverses the flow or can            where the suns effect is strongest. This has the effect of lowering the
create the conditions for shear failure by eroding soil adjacent to a            external footings. The doming is accentuated and cracking reduces
footing that runs in the same direction as the flow.                             or disappears where it occurred because of dishing, but other cracks
                                                                                 open up. The roof lines may become convex.
Saturation of clay foundation soil may occur where subfloor walls
create a dam that makes water pond. It can also occur wherever there             Doming and dishing are also affected by weather in other ways. In
is a source of water near footings in clay soil. This leads to a severe          areas where warm, wet summers and cooler dry winters prevail,
reduction in the strength of the soil which may create local shear               water migration tends to he toward the interior and doming will be
failure.                                                                         accentuated, whereas where summers are dry and winters are cold
                                                                                 and wet, migration tends to be toward the exterior and the
Seasonal swelling and shrinkage of clay soil affects the perimeter of
                                                                                 underlying propensity is toward dishing.
the building first, then gradually spreads to the interior. The swelling
process will usually begin at the uphill extreme of the building, or on          Movement caused by tree roots
the weather side where the land is flat. Swelling gradually reaches the          In general, growing roots will exert an upward pressure on footings,
interior soil as absorption continues. Shrinkage usually begins where            whereas soil subject to drying because of tree or shrub roots will tend
the sun's heat is greatest.                                                      to remove support from under footings by inducing shrinkage.

                                                                                 Complications caused by the structure itself
                                                                                 Most forces that the soil causes to he exerted on structures are
    Effects of Uneven Soil Movement on Structures                                vertical - i.e. either up or down. However, because these forces are
                                                                                 seldom spread evenly around the footings, and because the building
Erosion and saturation                                                           resists uneven movement because of its rigidity, forces are exerted
Erosion removes the support from under footings, tending to create
subsidence of the part of the structure under which it occurs.
Brickwork walls will resist the stress created by this removal of
support by bridging the gap or cantilevering until the bricks or the
                                                                                 from one part of the building to another. The net result of all these
                                                                                 forces is usually rotational. This resultant force often complicates the
                                                                                 diagnosis because the visible symptoms do not simply reflect the
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                                                                                 original cause. A common symptom is binding of doors on the
mortar bedding fail. Older masonry has little resistance. Evidence of
                                                                                 vertical member of the frame.
failure varies according to circumstances and symptoms may include:
                                                                                 Effects on full masonry structures
•     Step cracking in the mortar beds in the body of the wall or
                                                                                 Brickwork will resist cracking where it can. It will attempt to span
      above/below openings such as doors or windows.
                                                                                 areas that lose support because of subsided foundations or raised
•     Vertical cracking in the bricks (usually but not necessarily in line       points. It is therefore usual to see cracking at weak points, such as
      with the vertical beds or perpends).                                       openings for windows or doors.

Isolated piers affected by erosion or saturation of foundations will             In the event of construction settlement, cracking will usually remain

eventually lose contact with the bearers they support and may tilt or            unchanged after the process of settlement has ceased.

fall over. The floors that have lost this support will become bouncy,            With local shear or erosion, cracking will usually continue to develop
sometimes rattling ornaments etc.                                                until the original cause has been remedied, or until the subsidence
                                                                                 has completely neutralised the affected portion of footing and the
    Seasonal swelling/shrinkage in clay                                          structure has stabilised on other footings that remain effective.
    Swelling foundation soil due to rainy periods first lifts the most           In the case of swell/shrink effects, the brickwork will in some cases
    exposed extremities of the footing system, then the remainder of the         return to its original position after completion of a cycle, however it
    perimeter footings while gradually permeating inside the building            is more likely that the rotational effect will not be exactly reversed,
    footprint to lift internal footings. This swelling first tends to create a   and it is also usual that brickwork will settle in its new position and
    dish effect, because the external footings are pushed higher than the        will resist the forces trying to return it to its original position. This
    internal unes.                                                               means that in a case where swelling takes place after construction
                                                                                 and cracking occurs, the cracking is likely to at least partly remain
    The first noticeable symptom may be that the floor appears slightly
                                                                                 after the shrink segment of the cycle is complete. Thus, each time
    dished. This is often accompanied by some doors binding on the
                                                                                 the cycle is repeated, the likelihood is that the cracking will become
    floor or the door head, together with some cracking of cornice
                                                                                 wider until the sections of brickwork become virtually independent.
    mitres. In buildings with timber flooring supported by bearers and
joists, the floor can be bouncy. Externally there may be visible                 With repeated cycles, once the cracking is established, if there is no
    dishing of the hip or ridge lines.                                           other complication, it is normal for the incidence of cracking to
                                                                                 stabilise, as the building has the articulation it needs to cope with
    As the moisture absorption process completes its journey to the
                                                                                 the problem. This is by no means always the case, however, and
    innermost areas of the building, the internal footings will rise. If the
                                                                                 monitoring of cracks in walls and floors should always be treated
    spread of moisture is roughly even, it may be that the symptoms will
                                                                                 seriously.
    temporarily disappear, but it is more likely that swelling will be
    uneven, creating a difference rather than a disappearance in                  Upheaval caused by growth of tree roots under footings is not a
    symptoms. In buildings with timber flooring supported by bearers             simple vertical shear stress. There is a tendency for the root to also
    and joists, the isolated piers will rise more easily titan the strip          exert lateral forces that attempt to separate sections of brickwork
    footings or piers under walls, creating noticeable doming of flooring.        after initial cracking has occurred.